Supplemental oxygen controls cellular proliferation and anastomotic intimal hyperplasia at a vascular graft-to-artery anastomosis in the rabbit

Hemodynamics and Oxygen Transport through Pararenal Aortic Aneurysm Treated with Multilayer Stent: A Numerical Study

BACKGROUND

As opposed to an endoluminal stent graft, a multilayer stent (MS) consists of a porous mesh, which allows for the possibility of treating pararenal aortic aneurysms (PRAAs) that involve a significant branch vessel. However, the choice of the density of the MS plays a vital role in isolating the aneurysm and allowing unobstructed blood flow in the branch vessel.

METHOD

In the present study, we examined 3 cases (without a stent and with single-layer and double-layer stents) via numerical simulations to explore the feasibility of the MSs used in the treatment of such aneurysms and estimate whether there is a more appropriate or optimal stent density.

RESULTS

With stent intervention, the velocity of blood flow in the sac decreased, but the pressure on the surface of the aneurysm did not decrease although it became more uniform. In addition, the "region of double low" (with low wall shear stress and a low Sherwood number) enlarged after stent implantation. Even with the double-layer stent, however, the flux of the branch vessel was still above normal, and we could predict that the optimal stent porosity was approximately 49.9%.

CONCLUSIONS

Unlike in previous studies, an MS could not be feasibly applied to high-risk PRAAs. However, an MS can induce sac thrombosis in the later stages while maintaining visceral vessel patency, and our results suggest that the optimal stent may be a 4-layer stent.

Supplemental oxygen reverses hypoxia-induced smooth muscle cell proliferation by modulating HIF-alpha and VEGF levels in a rabbit arteriovenous fistula model

BACKGROUND

Numerous mechanisms for the formation of intimal hyperplasia have been proposed but none have been proven or accepted. Our research focuses on the potential role of hypoxia-inducible factors (HIFs), vascular endothelial growth factor (VEGF), and platelet-derived growth factors as well as the extracellular signal-regulated kinase (ERK), phosphatidylinositide 3-kinase /protein Kinase B (PI3-K/AKT) pathway in hypoxia-mediated intimal hyperplasia processes. We hypothesize that HIF and VEGF will be downregulated with supplemental oxygen in our arteriovenous fistula rabbit model.

METHODS

Rabbits were randomized into different experimental groups with varying oxygen exposure (21% O2 or 30% O2) and receipt of surgery (surgery with fistula formation, no surgery, or sham operation with skin incision only). Plasma samples were collected at designated intervals in which cytokines and smooth muscle cell proliferation were measured. In addition, cell specimens were exposed to hyperoxic, normoxic, and hypoxic environments with cytokines measured at various time points.

RESULTS

Placement of an arteriovenous fistula resulted in hypoxia-induced HIF stabilization with a concurrent increase in VEGF levels. There was a 4.2-fold induction in HIF-1α levels in animals that were placed in normal air after surgery when compared with animals that were exposed to hyperoxic air. Also, VEGF level significantly increased after surgery in the normoxic group, reaching a maximum of 959 pg/mL. Plasma VEGF levels in the surgery and supplemental oxygen group were significantly lower than the normoxic surgery group with almost a 45% reduction in plasma VEGF levels (524 pg/mL). Activation of VEGF receptors on smooth muscle cells through ERK1 and AKT pathways resulted in significant smooth muscle cell proliferation and migration. These effects are dramatically reduced in animals that are exposed to a hyperoxic environment of 30% oxygen.

CONCLUSIONS

Our results suggest that short-term administration of supplemental oxygen inhibits HIFs and VEGF signaling to reduce smooth muscle proliferation in the local blood vessel. These results provide strong support for the therapeutic use of supplemental oxygen after arterial surgery to reduce intimal hyperplasia. These findings also provide a nidus for future clinical trials to determine whether this is clinically applicable in humans.

The role of short-term oxygen administration in the prevention of intimal hyperplasia

OBJECTIVE

Intimal hyperplasia (IH) is the cause of most failed arteriovenous fistulas (AVFs), resulting in repeat procedures and leading to increased utilization of scarce health care resources. Our laboratory has previously demonstrated the role of supplemental oxygen in preventing IH and smooth muscle cell proliferation (SMCp) at an artery-to-graft anastomosis and at the deployment site of an intra-arterial stent. This study examines the effect of supplemental oxygen in preventing IH and SMCp in an AVF in a rabbit model.

METHODS

Ninety-six rabbits were randomized into four groups: group 1, control; group 2, no surgery with supplemental oxygen; group 3, AVF without supplemental oxygen; and group 4, AVF with supplemental oxygen. Rabbits receiving supplemental oxygen received 30% oxygen for up to 42 days. Specimens were collected in all groups at days 1, 3, 7, 21, 42, and 90. IH and SMCp were measured at the AVF site as well as in the artery and vein proximal and distal to the AVF.

RESULTS

IH was first noted at day 7 and significantly increased through day 90 at all locations in the nonoxygen-supplemented groups. No significant IH was noted in the oxygen-supplemented group at any location or any time point. SMCp was noted at day 3 through day 21 in the nonoxygen-supplemented group, whereas almost no SMCp was noted in the oxygen-supplemented group at any location or time point.

CONCLUSIONS

Without oxygen supplementation, SMCp begins at day 3 and is no longer noted at day 21 after creation of an AVF, whereas IH begins by day 7 and increases at least through day 90 after creation of an AVF. Forty-two days of 30% supplemental oxygen inhibits IH and SCMp after creation of an AVF. These data suggest a role for the short-term administration of low-dose O2 to prevent both IH and SMCp after creation of an AVF that may prolong patency and function.

Hypoxia differentially regulates arterial and venous smooth muscle cell proliferation via PDGFR-β and VEGFR-2 expression

Despite intensive research studies, theories have yet to focus on the contribution of hypoxia to patency differences observed clinically between arterial vs. venous grafts. This study investigates the differential hypoxic response of smooth muscle cells (SMC) to hypoxia-derived endothelial cell (EC) growth factors. Initiation of SMC proliferation under hypoxia (<5% O(2)) occurred only after incubation with hypoxic endothelial cell-conditioned media (H-ECM). After the investigation of several possible growth factors in the H-ECM that may be responsible for SMC proliferation, the greatest difference was observed in vascular endothelial growth factor (VEGF-A) and platelet-derived growth factor homodimer B (PDGF-BB) expression. VEGF-A increased (2-fold) significantly (P < 0.05) in arterial-derived smooth muscle cells (ASMC) under hypoxia compared with venous-derived smooth muscle cells (VSMC), which showed no significant change. VSMC showed significant (P < 0.05) increase in VEGFR-2 expression under hypoxia compared with ASMC. Incubation with VEGFR-2-neutralizing antibody/PDGFR antagonist in VSMC before addition of H-ECM resulted in decreased proliferation. ASMC proliferation under hypoxia did not decrease during incubation with VEGFR-2-neutralizing antibody but did decrease upon PDGFR antagonist incubation. Current therapies focusing on treating intimal hyperplasia have negated the fact that combinational therapy might be required to combat induction of SMC proliferation. Clinically, therapy with PDGFR antagonists plus anti-VEGFR-2 may prove to be efficacious in managing SMC proliferation in venous-derived grafts.

Arterial geometry, flow pattern, wall shear and mass transport: potential physiological significance

We have studied numerically steady and unsteady flow in a straight and a helically stented common carotid artery, in order to model porcine experimental results that show reduced intimal hyperplasia (IH) in the helical case. The combination of flow pulsatility and three-dimensionality generates a sweeping motion of the Dean vortices, which overall reduced extremes of both oxygen flux to the vessel wall and wall shear stress (WSS). Since IH and atherosclerosis affect preferentially low WSS regions, these findings imply that vessel three-dimensionality and flow pulsatility can play important protective roles in respect of these diseases. The amplitude and frequency of the velocity waveform are important parameters of the system. Increase in amplitude increases WSS and oxygen flux to the vessel wall. Increase in frequency has a small effect; it increases WSS but has no effect on the oxygen flux to the vessel wall.

Moderate hyperoxia (40%) increases antioxidant levels in mouse tissue

BACKGROUND

Oxygen is routinely administered to patients to improve clinical outcome. Since studies have shown that administering 100% oxygen can cause unwanted side effects, intermediate concentrations of 40% oxygen are used in clinical practice. In this study, we examined whether the breathing of 40% oxygen causes beneficial effects upon tissue levels of antioxidants such as vitamin E, vitamin C, and glutathione.

METHODS

Four-month-old mice were separated into two groups: control (n = 11) and experimental (n = 11). The treatment group was administered 40% oxygen for 10 days. Brain, heart, lung, liver, testes, and skeletal muscle were harvested and tissue antioxidant levels were determined by HPLC.

RESULTS

Vitamin E concentrations were higher in brain, heart, lung, liver, and testes of the treatment group (P < 0.05). Glutathione concentrations were higher in the lung tissue only (P < 0.05). No differences were found in vitamin C levels.

CONCLUSIONS

The data suggest that mice respond to oxidative stress by increasing tissue vitamin E incorporation and cellular synthesis of glutathione in the lung when exposed to moderate levels (40%) of hyperoxia.

Reoxygenation of hypoxic coronary smooth muscle cells amplifies growth-retarding effects of ionizing irradiation

BACKGROUND

Hypoxic human coronary smooth muscle cells (HCSMCs) are possible targets for brachytherapy to prevent restenosis after percutaneous transluminal coronary angiography. It is unclear whether growth kinetics and gene expression of these cells undergoing gamma-irradiation are changed by reoxygenation.

METHODS AND RESULTS

Hypoxic (H) and hypoxia-reoxygenated (H-R) HCSMCs were irradiated with gamma-radiation at single doses of 4, 8, and 16 Gy using a 60Co-source. Vascular endothelial growth factor gene expression of HCSMCs was dramatically suppressed in H-R versus H cells independent of the radiation dose (15+/-7% versus 2183+/-2023%, P<0.01, H-R versus H cells). An oxygen enhancement ratio of 1.8 was calculated after irradiation from the retarded growth of H-R versus hypoxic HCSMCs. Production of reactive oxygen species by HCSMCs after irradiation increased by 15+/-2% in H-R cells versus 7+/-1% in H cells (P<0.05).

CONCLUSIONS

Reoxygenation of hypoxic HCSMCs markedly amplifies growth-retarding effects of ionizing irradiation. On the basis of these findings, oxygenating radiosensitizers should be analyzed with regard to suitability for coronary brachytherapy to prevent restenosis.

Supplemental oxygen reduces intimal hyperplasia after intraarterial stenting in the rabbit

HYPOTHESIS

Supplemental oxygen can reduce intimal hyperplasia (IH) after stent deployment in a rabbit model.

BACKGROUND

Endovascular stent placement is technically feasible, but long-term durability in vessels outside the aortoiliac system is compromised with postinterventional IH, which causes restenosis and failure of the arterial conduit.

METHODS

Groups (n = 4 to 6) of female New Zealand white rabbits underwent placement of a 3-mm intraaortic stent with laparotomy and were placed in either normoxic (21% inspired oxygen concentration) or supplemental-oxygen (40% inspired oxygen concentration) environments for 0, 7, 14, and 28 days. The transarterial wall oxygen gradient was measured at 0, 7, and 28 days with an oxygen microelectrode. 5-Bromo-2'deoxyuridine (BrdU) was injected into the peritoneum before death to assess cellular proliferation. Aortic specimens were harvested en bloc and sectioned for analysis of cellular proliferation and intimal thickness.

RESULTS

Intraaortic stent placement significantly decreased the transarterial wall oxygen gradient in the outer 70% of the vessel wall and was easily reversed at 7, 14, and 28 days with application of supplemental oxygen. Cellular proliferation was significantly decreased at 14 days (0.5% +/- 0.001% versus 2.3% +/- 0.002%; P <.001) and 28 days (0.4% +/- 0.001% versus 1.0% +/- 0.001%; P <.025) as measured with count of nuclei staining for 5-Bromo-2'deoxyuridine in the intima and media. Intimal thickness was significantly decreased at 28 days in oxygen-supplemented rabbits (intimal area/medial area = 0.50 +/- 0.07) as compared with controls (intimal area/medial area = 0.89 +/- 0.11; P <.025).

CONCLUSION

This study shows the ability of supplemental oxygen to reverse arterial wall hypoxia, decrease cellular proliferation, and control IH at the deployment site of an intraarterial stent in a rabbit model. Forty-percent supplemental oxygen suppresses IH by 44% at 28 days as compared with normoxic control values. Cellular proliferation is reduced four-fold at 14 days and two-fold at 28 days in oxygen-supplemented rabbits as compared with control media after deployment. The clinical implications of these findings are significant, especially as the role of endovascular interventions continues to expand.